Origin of ENDOGENOUS RETROVIRUSES?

Question

Biologists have concluded that primates were invaded millions of years ago by some of the endogenous retroviruses.
OK.
But before that, where did those retroviruses come from. Did they originate in some other non primate animal. If they were from some other non primate animal, does that mean they caught it from yet some other animal.
Was there some animal that had it on its own without catching it from other animals.

Answer 1

There is little known about the origins of viruses since there are no fossil viruses. But here is a page that discusses the broad aspects of the rise from the early seas.
http://www.mcb.uct.ac.za/tutorial/virorig.html
"Viruses of the major classes of organisms - animals, plants, fungi and bacteria / archaea - probably evolved with their hosts in the seas & emerged from the waters with their different hosts, during the successive waves of colonisation of the terrestrial environment. This explains the extreme host specificity as like all good parasites they co-evolved with their host." They have had aeons to adapt to each "life niche".

Endo-genous means they are now part of the genome but they were once exo-genous or just retroviruses (like the AIDS virus) infecting sequential hosts in one early primate species. As the host changed so did the retrovirus. If the host speciated the retrovirus would also be carried along into each new species as it has been since the first host in the sea.
However the insertion of each retrovirus infection is random so if the retrogenes are found in identical chromosomal positions of two different species it indicates common ancestry not cross infection.

Viruses have they ability to become incorporated in the host DNA then re-emerge at a later date. This insertion into the host DNA is called endogenization. Some come in to take over the cell to make more viruses immediately while others rest a while as just another bit of DNA. This way as the cell divides there are still more viruses being made. Endogenous retroviruses (ERVs) then get copied by the host DNA into daughter cells so are carried into another generation if in germ cells.
There is a flaw in this for the virus. There was no selective pressure to keep them from copy errors so most ERV sequences accumulated numerous nonsense mutations over time, rendering them defective. They can no longer assemble as active viral particles but a few genes are still expressed as proteins. Since the reading frames have remained open there must be positive selection for those genes. They must offer a benefit because in all it seems we are using 16 genes from this source. These came from a possible 200,000 -250,000 retroviral insertions that mutated but left these few functional genes as an inadvertent bequest.